Noise When Reversing

[Big P UK300]

Going to take the car to the garage sometime next week to have this checked out , but was wondering if anybody had an idea what it might

be ? Occasionally when reversing or pulling away slowly , im getting a noise from the rear end that can be best described as someone

blowing over the top of a bottle (if that makes sense) A sort deep pitched squeak

Any thoughts ? Thank's

Paul